The 70-year-old guitarist is battling Parkinson's disease, and on Tuesday (13.02.18) the group announced that while he was still a member of the band, he would be taking time away from life on the road.
However, Rob says that if his bandmate's condition improves, there is always the possibility he could be fit enough to return to the stage.
Speaking about the decision, frontman Rob told The Quietus: ''We're still kind of digesting all of this.
''Glenn made the decision about not being able to full commit to the tour only last Thursday (08.02.18).
''So we're still very much processing all of the possibilities.
''The other thing is, much like some of the other areas in these kinds of challenges, daily, there are incredible advances in medicines, in drugs and such things that can help to alleviate the condition. So that's all part of the ongoing story for Glenn.
''Who knows? Next year or the year after that, things could be dramatically different.
''But in the meantime, yeah, Glenn can show up, as we've said in the statement, anytime anywhere.''
Rob also repeated the sentiment in the band's statement which said there was still a chance Glenn would join them for special appearances on the tour.
He added: ''Glenn is still Glenn - Glenn can still play the guitar. Same guy - a wonderful, great heavy metal guitar player. His role is a little bit different now.
''And I can't wait for those moments when he's gonna show up unexpectedly anywhere around the world on the 'Firepower' tour and come and jam out 'Living After Midnight', 'Breaking The Law', those kinds of songs.
''It's gonna be a really special moment. It could be the first show, it could be the last show - we don't know. But the joy is that Glenn is still playing and he's still with us, and he's as excited as we are to celebrate the release of 'Firepower'.''